In situ preparation of a bimorphological latex

ABSTRACT

The present invention relates to an in situ method for preparing a bimorphological aqueous dispersion of first polymer particles with protuberating phosphorus acid cores and second polymer particles without protuberating cores. The method provides a more efficient way of making compositions for pigmented coating formulations.

The present invention relates to the in situ preparation of an aqueousdispersion of polymer particles with two distinct morphologies, i.e., abimorphological latex. This latex is useful as a binder in coatingsformulations.

Titanium dioxide (TiO₂) is an expensive component in many pigmentedcoatings formulations. The efficacy of TiO₂ as a hiding pigment isreduced when TiO₂ particles are allowed to come too close together uponfilm formation and drying (which they tend to do). The spacing of TiO₂and its resultant efficiency can be improved using an adsorbing emulsionpolymer. For example, U.S. Pat. No. 7,179,531 (Brown et al.) discloses adispersion of multistage polymer particles characterized by a relativelysmall core portion protuberating from a relatively large shell portionof each particle, with the core portion being preferentiallyfunctionalized with TiO₂-adsorbing groups, typically phosphorus acidfunctionalized groups. These so-called “acorn” particles are disclosedas being useful for preparing TiO₂-polymer composite particles thatprovide dried coatings with improved hiding.

More recently, US 2015/0005446A1 (Bohling et al.) demonstrated a way ofpreferentially focusing the phosphorus acid functionality to theprotuberating core portion of the acorn particle, thereby reducingflocculation that can occur from inadvertent phosphorus acidincorporation at the surface of the shell portion.

US 2015/0011695 (Bohling et al.) describes a bimodal adsorbing latexdesigned to increase solids content while retaining the advantages ofhiding achieved with the adsorbing polymer particle technology.

The discovery of TiO₂-adsorbing polymer particles to improve hidingefficiency represents a major advance in coatings technology;nevertheless, inasmuch as current processes require that TiO₂ particlesbe combined with far more adsorbing polymer particles than is necessaryto maximize spacing between the particles, the flexibility of usage ofthe relatively inexpensive letdown binder is limited. It would thereforebe an advantage in the art of coating compositions to discover a way toboth optimize spacing between pigment particles and, at the same time,provide a way to “dial in” the desired amount of non-adsorbing letdownbinder.

SUMMARY OF THE INVENTION

The present invention provides an advance in the art of coatingcompositions by providing an in situ method for preparing abimorphological aqueous dispersion of polymer particles comprising thesteps of:

a) mixing together an aqueous dispersion of phosphorusacid-functionalized acrylic-based first seed polymer particles having avolume average particle size in the range of from 40 nm to 85 nm with anaqueous dispersion of acrylic-based second seed polymer particlescomprising a substantial absence of phosphorus acid functionality andhaving a volume average particle size in the range of from 20 nm to 80nm;b) contacting monomers with the aqueous dispersions of the first andsecond seed polymer particles under emulsion polymerization conditionsto form a dispersion of 1) polymer particles with a protuberatingphosphorus acid functionalized core; and 2) polymer particles without aprotuberating core; andwherein the weight-to-weight ratio of particles with the protuberatingphosphorus acid functionalized core to particles without a protuberatingcore is in the range of 25:75 to 75:25.

The present invention provides an efficient one-step pathway forcontrolling the relative amounts of TiO₂-adsorbing and non-adsorbingpolymer particles for pigmented coating formulations.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an in situ method for preparing abimorphological aqueous dispersion of polymer particles comprising thesteps of:

a) mixing together an aqueous dispersion of phosphorusacid-functionalized acrylic-based first seed polymer particles having avolume average particle size in the range of from 40 nm to 85 nm with anaqueous dispersion of acrylic-based second seed polymer particlescomprising a substantial absence of phosphorus acid functionality andhaving a volume average particle size in the range of from 20 nm to 80nm;b) contacting monomers with the aqueous dispersions of the first andsecond seed polymer particles under emulsion polymerization conditionsto form a dispersion of 1) polymer particles with a protuberatingphosphorus acid functionalized core; and 2) polymer particles without aprotuberating core; andwherein the weight-to-weight ratio of particles with the protuberatingphosphorus acid functionalized core to particles without a protuberatingcore is in the range of 25:75 to 75:25.

As used herein, the term “acrylic-based” refers to polymer particles(including seed polymer particles) that comprise at least 30 weightpercent, based on the weight of the polymer particles, structural unitsof one or more methacrylate monomers such as methyl methacrylate andethyl methacrylate, and/or one or more acrylate monomers such as ethylacrylate, butyl acrylate, 2-propylheptyl acrylate, and 2-ethylhexylacrylate. The acrylic-based polymers may also include structural unitsof other non-acrylate or methacrylate monomers such as styrene.

As used herein, the term “structural unit” of the named monomer refersto the remnant of the monomer after polymerization. For example, astructural unit of methyl methacrylate is as illustrated:

where the dotted lines represent the points of attachment of thestructural unit to the polymer backbone.

The aqueous dispersion of the first seed polymer particles isadvantageously prepared by emulsion polymerization of monomerscomprising a) methyl methacrylate or styrene or a combination thereof,preferably methyl methacrylate; b) one or more acrylates selected fromthe group consisting of ethyl acrylate, butyl acrylate, 2-propylheptylacrylate, and 2-ethylhexyl acrylate; and c) a phosphorus acid monomer ora salt thereof. Examples of suitable phosphorus acid monomers includephosphonates and dihydrogen phosphate esters of an alcohol in which thealcohol contains or is substituted with a polymerizable vinyl orolefinic group. Preferred dihydrogen phosphate esters are phosphates ofhydroxyalkyl acrylates or methacrylates, including phosphoethylmethacrylate (PEM) and phosphopropyl methacrylates. PEM, which is anespecially preferred phosphorus acid monomer, is represented by thefollowing structure:

where R is H or

A carboxylic acid monomer or a sulfur acid monomer, or salts thereof orcombinations thereof are preferably included in the emulsionpolymerization of the first seed polymer particles. Examples of suitablecarboxylic acid monomers include acrylic acid, methacrylic acid, anditaconic acid, and salts thereof; examples of suitable sulfur acidmonomers include sulfoethyl methacrylate, sulfopropyl methacrylate,styrene sulfonic acid, vinyl sulfonic acid, and 2-acrylamido-2-methylpropanesulfonic acid, and salts thereof.

A multiethylenically unsaturated monomer such as allyl methacrylate ordivinyl benzene is more preferably included in the emulsionpolymerization of the first seed polymer particles.

Preferably, the first seed polymer particles comprise, based on theweight of the first seed polymer particles: a) 2 to 12 weight percentstructural units of a phosphorus acid monomer or a salt thereof; b) from0.5 to 20 weight percent structural units of a carboxylic acid monomeror a sulfur acid monomer or a salt thereof or a combination thereof; c)from 0.1 to 30 weight percent structural units of a multiethylenicallyunsaturated monomer; and d) a sufficient concentration of structuralunits of one or more polymerizable ethylenically unsaturated bulkmonomers so that the pre-formed polymer particles have a T_(g), ascalculated by the Fox equation, in the range of from −50° C., preferablyfrom −40° C., more preferably from −20° C.; to 75° C., preferably to 30°C., and more preferably to 20° C.

More preferably, the first seed polymer particles comprise, based on theweight of the first seed polymer particles: a) 3 to 8 weight percentstructural units of PEM or a salt thereof; b) from 1 to 5 weight percentstructural units of acrylic acid or methacrylic acid or a salt thereof;c) from 0.2 to 5 weight percent structural units of allyl methacrylate;d) from 50 to 65 weight percent structural units of butyl acrylate; ande) 25 to 45 weight percent structural units of methyl methacrylate.

The volume average particle size of the first seed polymer particles, asmeasured by a BI-90 Plus particle size analyzer, is from 40 nm,preferably from 45 nm, more preferably from 55 nm, and most preferablyfrom 60 nm, to 85 nm, preferably to 80 nm.

The aqueous dispersion of the second seed polymer particles isadvantageously prepared by emulsion polymerization of a) methylmethacrylate or styrene or a combination thereof, preferably methylmethacrylate; b) one or more acrylates selected from the groupconsisting of ethyl acrylate, butyl acrylate, 2-propylheptyl acrylate,and 2-ethylhexyl acrylate; and c) a substantial absence of a phosphorusacid monomer.

As used herein, the term “a substantial absence of a phosphorus acidmonomer” refers to not more than 1 weight percent, preferably not morethan 0.5 weight percent, more preferably not more than 0.1 weightpercent, and most preferably 0 weight percent phosphorus acid monomer,based on the weight of the second seed polymer particles.

A carboxylic acid monomer or a sulfur acid monomer, or salts thereof orcombinations thereof are preferably included in the emulsionpolymerization of the second seed polymer particles.

Preferably, the monomers comprise, based on the weight of the monomers:a) methyl methacrylate or styrene or a combination thereof; b) one ormore acrylate monomers selected from the group consisting of ethylacrylate, butyl acrylate, 2-propylheptyl acrylate, and 2-ethylhexylacrylate; and c) a substantial absence of a phosphorus acid monomer,based on the weight of the monomers.

More preferably the monomers comprise: a) from 40, more preferably from45, to 60, more preferably to 55 weight butyl acrylate or 2-ethylhexylacrylate or a combination thereof; b) from 40, more preferably from 45,to 60, more preferably to 55 weight percent methyl methacrylate orstyrene or a combination there; c) from 0.1 to 5 weight percent acrylicacid or methacrylic acid or sodium 4-vinylbenzenesulfonate or2-acrylamido-2-methyl propanesulfonic acid or salts thereof orcombinations thereof; e) from 0.1 to 5 weight percent ureidomethacrylate; and f) 0 weight percent of a phosphorus acid monomer.

The volume average particle size of the second seed polymer particles,as measured by a BI-90 Plus particle size analyzer, is from 20,preferably from 30 nm, more preferably from 40 nm, to 80 nm, preferablyto 70 nm, and more preferably to 60 nm.

The aqueous dispersions of the first and second seed polymer particlesare mixed together at a first-to-second seed polymer particlesweight-to-weight ratio of preferably from 1:2, more preferably from 1:1,to preferably 10:1, more preferably 5:1, and most preferably to 3:1. Themixture is then contacted with the monomers, preferably an aqueousemulsion of the monomers under emulsion polymerization conditions toform polymer particles with a protuberating phosphorus acidfunctionalized core and polymer particles without a protuberating core.

Preferably, the monomers have the same monomer profile as the secondseed polymer particles, which means that the monomers that are used toprepare the second seed polymer are preferably the same and in the sameproportions as the monomers fed to the reactor. The subsequently formedshells of the protuberating polymer particles and the particles withoutprotuberating cores have substantially identical compositions: They areidentical when the monomer profile of the monomers is the same as thesecond seed polymers, and almost identical when the profiles aredifferent.

In a most preferred method of making the bimorphological composition, aportion of the monomer emulsion (˜1 to 10 weight percent of the totalmonomers in the monomer emulsion) is polymerized under emulsionpolymerization conditions to form the aqueous dispersion of the secondseed polymer particles. Alternatively, the first and second seed polymerparticles can be formed independently in separate reactors. In eithercase, the first and second seeds must have different compositions toform the bimorphological latex. The aqueous dispersions of the first andsecond seed polymer particles are then combined, followed by addition ofthe remainder of the monomer emulsion, followed by emulsionpolymerization. The concentration of first seed polymer particles ispreferably from 2 to 10 weight percent based on the weight of the totalmonomers in the subsequently added monomer emulsion.

The resultant dispersion of bimorphological polymer particles, that is,polymer particles with and without a phosphorus acid functionalizedprotuberating core, preferably have a weight-to-weight ratio ofparticles with a protuberating core to particles without a protuberatingcore in the range of from 30:70, more preferably from 50:50, mostpreferably from 55:45, to preferably 68:32, and more preferably to65:35. The volume average particle size of the polymer particles withthe protuberating core is in the range of from 110, preferably from 120nm to 160, preferably to 150 nm; and the volume average particle size ofthe particles without a protuberating core is in the range of from 70,more preferably from 80, and most preferably from 85 nm, to 140, morepreferably to 130 nm, as determined by Asymmetric Flow Field FlowFractionation. Preferably, the ratio of the volume average particle sizeof the protuberating core particles to the non-protuberating coreparticles is from 1.45:1, more preferably from 1.42:1 to 0.9:1, morepreferably to 1:1, more preferably to 1.1:1, and most preferably to1.2:1.

The aqueous dispersion of bimorphological particles is advantageouslycombined with pigment particles such as TiO₂ particles to form adispersion of first polymer particles, as least some of which adsorb tothe TiO₂ particles, and second polymer particles, at least some of whichdo not adsorb to the TiO₂ particles. Accordingly, the present inventionprovides an efficient way to prepare coating compositions with desiredlevels of adsorbing and letdown binder. The coating composition furtheradvantageously includes one or more of the following components:defoamers, surfactants, dispersants, rheology modifiers, andneutralizing agents.

EXAMPLES

Measurements of Particle Size and Weight Ratios Using AFFFF

AFFFF flow regulation was controlled using Eclipse 3+ (WyattTechnology). Data from UV and multiangle light scattering (MALS, DAWNHELEOS, Wyatt Technology) detectors were collected and processed byAstra 6.1.2.76 software (Wyatt Technology). Latex samples were diluted1000-fold and 100-nm and 150-nm polystyrene NIST traceable particle sizestandards (Nanosphere standards from Thermo Scientific) were diluted100-fold with purified water. The separation channel dimensions were15.2 cm in length and tapered from 2.15 to 0.3 cm in width, with a350-μm thickness spacer, and an ultrafiltration membrane regeneratedcellulose with a 10-kDa cutoff (Wyatt Technology). The 90-degree MALSdetector was calibrated with HPLC grade toluene; the detectors at otherangles were normalized using the peak maximum of the 100-nm standardusing the sphere model.

Resolution and fractionation power, which measure the degree ofseparation between two components, can be determined based on theelution profile for the standards using equations disclosed in Schimpf,M. E., Resolution and Fractionating Power. in Field-Flow FractionationHandbook, Schimpf, M.; Caldwell, K.; Giddings, J. C., Eds. WileyInterscience: New York, 2000; pp 71-94. Resolution (R_(s)) of componentsis defined by the following equation:

$R_{s} = \frac{\delta\; t_{r}}{4{\overset{\_}{\sigma}}_{t}}$

Where δt_(r) is the difference in retention time between the twocomponents and σ _(t) is the average standard deviation of the twocomponent zones in units of time.

Diameter based Fractionating Power (F_(d)) is the resolution betweenparticles whose average diameter (d) differ by the relative incrementδd/d, as calculated by the following equation:

$F_{d} = \frac{R_{s}}{\delta\;{d/d}}$

The Resolution for the standards should be ≧2 and the FractionatingPower should be ≧5. For the 100-nm and 150-nm polystyrene standards usedin the present analysis, the Resolution and Fractionating Power were 2.2and 5.6 respectively.

The mobile phase used for AFFFF analysis was 0.1% of Fisherbrand FL-70solution (Fisher Scientific). The following flow rates were used for allsamples: Detector Flow: 0.7 mL/min; Focus Flow: 2 mL/min; Inject Flow:0.2 mL/min.

Initial hold time before injection: 1 min, 0 crossflow; focus: 1 min;focus and injection: 1 min; focus: 1 min; elution: 10 min at 1 mL/mincrossflow; elution: 30 min ramped crossflow from 1 mL/min to 0.5 mL/min;elution 6 min, 0 crossflow.

Latex samples were diluted 1000-fold with purified water prior toinjection (20 μL) into the AFFFF analyzer for characterization. The MALSdata collection rate was 1 s/data point. Purified water also injectedinto the analyzer to obtain blanks for UV baseline subtraction. Latexparticle sizes were obtained by MALS using the sphere model. Thearea:area ratio of particle sizes (assumed to be the same asweight:weight ratio) were determined by fitting overlapping UV curves(220 nm) into two Gaussian peaks.

Example 1—Preparation of Bimorphological Polymer Particles

A. Core (Preform) Synthesis

A first monomer emulsion was prepared by mixing deionized water (200 g),Disponil FES 993 surfactant (43 g, 30% active), butyl acrylate (371.2g), methyl methacrylate (195.2 g), allyl methacrylate (9.6 g),phosphoethyl methacrylate (51.2 g, 60% active), and methacrylic acid(12.8 g).

To a 5-L, four necked round bottom flask equipped with a paddle stirrer,a thermometer, nitrogen inlet, and a reflux condenser was addeddeionized water (600 g) and Disponil FES 32 surfactant (43 g, 30%active). The contents of the flask were heated to 85° C. under N₂ andstirring was initiated. A portion of the first monomer emulsion (70 g)was then added, quickly followed by a solution of sodium persulfate(2.56 g) dissolved in deionized water (30 g) followed by a rinse ofdeionized water (5 g). After stirring for 10 min, the remainder of thefirst monomer emulsion, followed by a rinse (25 g), and an initiatorsolution of sodium persulfate (0.64 g) dissolved in deionized water (50g) were added linearly and separately over 40 min. After the monomeremulsion feed was complete, the contents of the flask were held at 85°C. for 10 min, after which time the co-feed was complete; and thecontents of the flask were then held at 85° C. for an additional 10 min.The contents of the flask were cooled to room temperature andneutralized to pH 3 with a dilute solution of ammonium hydroxide. Themeasured particle size using a Brookhaven BI 90 Plus particle analyzerwas 60-75 nm and the solids were 40%.

B. Acorn Core-Shell Synthesis

A second monomer emulsion was prepared using deionized water (400 g),sodium dodecylbenzene sulfonate (55.4 g, 23% active), Disponil FES 993surfactant (48.17 g, 30% active), butyl acrylate (775.2 g), methylmethacrylate (797.33 g), ureido methacrylate (44.2 g, 50% active),acrylic acid (10.2 g), and sodium 4-vinylbenzenesulfonate (11.33 g, 90%active).

To a 5-L, four necked round bottom flask equipped with a paddle stirrer,a thermometer, N₂ inlet, and a reflux condenser was added deionizedwater (850 g) and Disponil FES 993 surfactant (5.65 g, 30% active). Thecontents of the flask were heated to 84° C. under N₂ and stirring wasinitiated. A portion of the second monomer emulsion (75 g, 3.5% of totalmonomer) was then added, quickly followed by an aqueous solution ofammonium persulfate (5.1 g) dissolved in deionized water (25 g) followedby a rinse of deionized water (5 g). After stirring for 10 min, aportion of the pre-form from Step A was then added (212.5 g 5.0% oftotal monomer), followed by addition of the remainder of the secondmonomer emulsion and then a solution containing ammonium persulfate (1.7g) and ammonium hydroxide (5 g, 29% active) dissolved in deionized water(55 g), each added linearly and separately to the flask over a totalperiod of 80 min. The contents of the flask were maintained at atemperature of 84° C. during the addition of the second monomeremulsion. When all additions were complete, the flask containing thesecond monomer emulsion was rinsed with deionized water (25 g), whichwas then added to the flask.

The contents of the flask were cooled to 65° C. and a catalyst/activatorpair was added to the flask to reduce residual monomer. TERGITOL™15-S-40 surfactant (12.15 g, 70% solids) was added. The polymer was thenneutralized to pH 9 with a dilute ammonium hydroxide solution. Theparticle sizes, as measured by Asymmetric Flow Field Flow Fractionation(AFFFF), were 109 nm for the non-protuberating polymer particles and 139nm for the protuberating core polymer particles; the solids were 49.5%.

Examples 2-6

The procedure used to prepared compositions of Examples 2-6 wassubstantially the same as described in Example 1 except that theportions of the second monomer emulsion and the pre-form were varied toachieve different w/w ratios of non-protuberating to protuberatingparticle particles, different particle sizes. Table 1 illustratesamounts of perform and second monomer emulsion (ME2) as a percentage ofthe total monomer used to prepare the particles; the particle sizes (PS)of the particles, and the w/w ratio of the non-protuberating toprotuberating particles. If desired, particle size and particle sizeratios can be further manipulated through control of surfactantconcentration in the reactor at the onset of polymerization.

TABLE 1 Amounts, Particle Sizes, and w/w Ratios of BimorphologicalPolymer Particles Non-protub Protub Ex. No. % Preform % ME2 PS (nm) PS(nm) Ratio NonPro:Pro 1 5.0 3.5 109 139 53.1:46.9 2 2.5 3.5 117 14770.3:29.7 3 7.5 3.5 103 134 40.1:59.9 4 10.0 3.5 89 125 34.0:66.0 5 5.02.5 102 138 46.7:53.3 6 5.0 4.5 111 138 62.0:38.0

Paint formulations were prepared using the bimorphological latexes andfound to have nearly identical properties (for example, hiding, KUstability, gloss, heat age stability, and rheology modifier demand) aspaints formulated with separately added latexes with the differentmorphologies, demonstrating that the added efficiency in the preparationof the composition of the present invention does not cause adverseeffects in final properties of the formulated paint.

The invention claimed is:
 1. An in situ method for preparing abimorphological aqueous dispersion of polymer particles comprising thesteps of: a) mixing together an aqueous dispersion of phosphorusacid-functionalized acrylic-based first seed polymer particles having avolume average particle size in the range of from 40 nm to 85 nm with anaqueous dispersion of acrylic-based second seed polymer particlescomprising a substantial absence of phosphorus acid functionality andhaving a volume average particle size in the range of from 20 nm to 80nm; b) contacting monomers with the aqueous dispersions of the first andsecond seed polymer particles under emulsion polymerization conditionsto form a dispersion of 1) polymer particles with a protuberatingphosphorus acid functionalized core; and 2) polymer particles without aprotuberating core; and wherein the weight-to-weight ratio of particleswith the protuberating phosphorus acid functionalized core to particleswithout a protuberating core is in the range of 25:75 to 75:25.
 2. Themethod of claim 1 wherein the phosphorus acid-functionalizedacrylic-based first seed polymer particles comprise structural units of:a) methyl methacrylate or styrene or a combination thereof; b) one ormore acrylate monomers selected from the group consisting of ethylacrylate, butyl acrylate, 2-propylheptyl acrylate, and 2-ethylhexylacrylate; and c) phosphoethyl methacrylate; and wherein the monomerscomprise: a) methyl methacrylate or styrene or a combination thereof; b)one or more acrylate monomers selected from the group consisting ofethyl acrylate, butyl acrylate, 2-propylheptyl acrylate, and2-ethylhexyl acrylate; and c) a substantial absence of a phosphorus acidmonomer, based on the weight of the second seed polymer particles. 3.The method of claim 2 wherein the phosphorus acid-functionalizedacrylic-based first seed polymer particles further comprise: d)structural units of a carboxylic acid monomer or a salt thereof; and e)structural units of a multiethylenically unsaturated monomer.
 4. Themethod of claim 3 wherein the monomers further comprises a carboxylicacid monomer or a sulfur acid monomer or a salt thereof or a combinationthereof.
 5. The method of claim 4 wherein the phosphorusacid-functionalized acrylic-based first seed polymer particles comprise,based on the weight of the first seed polymer particles: a) 3 to 8weight percent structural units of phosphoethyl methacrylate or a saltthereof; b) from 1 to 5 weight percent structural units of acrylic acidor methacrylic acid or a salt thereof; c) from 0.2 to 5 weight percentstructural units of a multiethylenically unsaturated monomer; d) from 50to 65 weight percent structural units of butyl acrylate; and e) 25 to 45weight percent structural units of methyl methacrylate; and wherein themonomers are added as a monomer emulsion and comprises, based on theweight of the monomers: a) from 40 to 60 weight percent butyl acrylateor 2-ethylhexyl acrylate or a combination thereof; b) from 40 to 60weight percent methyl methacrylate or styrene or a combination thereof;c) from 0.1 to 5 weight acrylic acid or methacrylic acid or sodium4-vinylbenzenesulfonate or 2-acrylamido-2-methyl propanesulfonic acid ora salt thereof or a combination thereof; and d) less than 0.5 weightpercent of a phosphorus acid monomer.
 6. The method of claim 5 whereinthe monomers have the same monomer profile as the second seed polymerparticles.
 7. The method of claim 1 wherein the aqueous dispersions ofthe first and second seed polymer particles are mixed together at afirst-to-second seed polymer particles weight-to-weight ratio of from1:2 to 10:1, wherein the monomer emulsion comprises less than 0.1 weightof a phosphorus acid monomer based on the weight of the monomeremulsion.
 8. The method of claim 7 wherein the ratio of volume averageparticle size of the protuberating core polymer particles to thenon-protuberating core polymer particle is from 1.45:1 to 0.9:1.
 9. Themethod of claim 8 wherein the volume average particle size of theprotuberating core polymer particles is in the range of from 110 to 160nm, and the volume average particle size of the non-protuberating corepolymer particles is in the range of from 70 to 140 nm.
 10. The methodof claim 9 wherein the ratio of volume average particle size of theprotuberating core polymer particles to the non-protuberating corepolymer particle is from 1.42:1 to 1.2:1; the volume average particlesize of the protuberating core polymer particles is in the range of 110to 150 nm, and the volume average particle size of the non-protuberatingcore polymer particles is in the range of 80 to 130 nm.
 11. The methodof claim 10 wherein weight-to-weight ratio of particles with theprotuberating phosphorus acid functionalized core to particles without aprotuberating core is in the range of 50:50 to 68:32.
 12. The method ofclaim 1 which comprises the further step of combining thebimorphological aqueous dispersion of polymer particles with TiO₂particles to form a dispersion polymer particles that are adsorbed andnot adsorbed to TiO₂.